1. Field of the Invention
The present invention relates to a constitution which performs a processing of an image for medical purposes.
2. Related Background Art
In recent years, with the advancement in digital techniques, there has been performed a processing, for example, which comprises: receiving radiation (X rays, and the like) by a sensor via an object; digitizing a resulting radiation image; performing a predetermined image processing on a digital image; and transmitting an output to a monitor, or a film for X-ray diagnosis.
Examples of the image processing include a gradation conversion processing in which gradation conversion is performed in accordance with density distribution of an original image obtained by radiographing. By the gradation conversion processing, the image obtained by the radiographing is converted to a density value which is easily observed, and outputs are transmitted to the monitor, the film, and the like.
For example, FIG. 29 shows an X-ray image 600 which is obtained by X-ray photographing of a cervical vertebra front part. In FIG. 29, an area 601 (black portion) shows a head part, and the other area 602 shows a throat part.
Here, when the X-ray image 600 is outputted to the film for X-ray diagnosis, first, a characteristic amount of the X-ray image 600 is extracted. In this case, in a method of extracting the characteristic amount, as shown in FIG. 30, used is a histogram of a remaining area which is obtained by deleting a passing through area (area in which X rays are directly radiated to the sensor) from the X-ray image 600. In FIG. 30, abscissa shows pixel value, and ordinate shows output frequency. Therefore, in the histogram, the pixel value (x) of a lower portion, for example, a constant portion (low density portion) point such as 5% point is extracted as the characteristic amount, and the density value of the X-ray image 600 is converted so that the extracted pixel value (x) reaches a density value of about 1.0 on the film.
However, in the above-described conventional characteristic amount extracting method, the pixel value around the low density portion at most is merely extracted as the characteristic amount for use in the gradation conversion processing. Specifically, the portion from which the characteristic amount is extracted is not necessarily stable. Therefore, when the characteristic amount obtained in the conventional method is used to perform the gradation conversion processing on the photographed image, there arises a problem that the density value is dispersed in the film or the like for each photographed image. This deteriorates the image quality of the processed image.